The performance of a commercial transport airplane is typically measured in terms of mission capability and operating costs. Mission capability can be improved by reducing airplane drag during takeoff climb and cruise, and by utilizing designs that minimize structural weight. Operating costs can be reduced by reducing airplane cruise drag (hence, resulting in less fuel burn and less fuel costs) and by utilizing designs that are inexpensive to manufacture and maintain. Further, for commercial operators, higher profits can be achieved by being able to transport more customers and/or goods for a given flight. Because the additional payload increases takeoff weight, it is even more desirable to reduce takeoff drag for takeoff-climb-limited missions.
The objectives of reducing drag, reducing weight, and reducing complexity (hence manufacturing and maintenance costs) are often in conflict. Adding a wingtip extension member can reduce the drag of a given airplane, but this will usually require increasing structural weight. Weight increases are due to the weight of the wingtip extension member and also due to strengthening required of the existing wing structure in order to support the increased bending moments exerted by the wingtip extension member. Additional weight penalties can also occur if the extension exacerbates flutter.
This conflict between the benefits of reduced drag and the disadvantages of increased weight has motivated designers to find an optimal balance between the two when designing a wingtip extension member. One such attempt is described in U.S. Pat. No. 5,039,032, hereinafter the '032 patent, incorporated herein by reference. The '032 patent describes a number of wingspan extensions termed "High Taper Wing Tip Extensions". These are also known as "raked wingtips". Raked wingtips are generally characterized by leading-edge sweep angles that are greater than the main wing sweep angles and are significantly tapered (i.e., the chord length decreases in the spanwise direction.)
Raked wingtips offer several advantages, some of which are outlined in the '032 patent. These advantages include the aerodynamic benefit of drag reduction due to increased wingspan, and a number of weight-reduction advantages (relative to simply extending the wingspan of an existing conventional main wing.) Two weight advantages are attributed to the wingtip taper. At high-load-factor structural design conditions, the smaller chords are subjected to less load and they result in less induced loading on the outboard main wing. These are both factors that reduce the bending moment that the inboard wing must support. Two more weight advantages are attributed to leading-edge sweep. The leading-edge sweep of a raked wingtip results in the center of pressure being located further aft than for a simple extension of an existing conventional main wing. At the high load-factor structural design conditions, this relative aft-movement of the center of pressure causes the sections of the main wing adjacent to the raked wingtip to be twisted more leading-edge-down, thus reducing the loading on these sections and the bending moment that the inboard wing must support. The relative aft-movement of the center of pressure also acts to attenuate flutter. The raked wingtips described in patent '032 range from moderate span extensions (e.g., 6% increase in span) to large span extensions (e.g., 12% increase in span). It is the large span extensions that offer the greatest benefits.
Regardless of these benefits, there are challenges in implementing raked wingtips on some aircraft. For example, on aircraft designed to operate at high subsonic Mach numbers (i.e., at or greater than about 0.70) there is a tendency for the boundary layer on the upper surface of each raked wingtip to separate under high-lift conditions (such as during takeoff climb or landing). This boundary-layer separation has the potential to increase drag and to generate premature buffet. The primary motivation for adding a wingspan extension is to increase the lift-to-drag ratio (primarily by decreasing drag), both during cruise and takeoff climb. If there is a significant drag increase due to large-scale boundary-layer separation under takeoff climb conditions, part or all of the takeoff-climb improvement is lost. When the raked wingtip boundary layer separates, there is also a possibility of unsteady aerodynamic forces strong enough to vibrate the airplane structure and to be perceived by the airplane pilot as buffet indicating the onset of aerodynamic wing stall. If this form of buffet occurs prematurely (that is, within what would normally be the operating envelope), stall speed must be declared at a speed significantly higher than the aerodynamic wing stall, thus degrading airplane performance.
The '032 patent acknowledges the tendency of the boundary layers on raked wingtips to separate under high-lift conditions. In the '032 patent, raked wingtips are categorized into two groups, one group with leading-edge sweep angles between 40 and 50 degrees and another with leading-edge sweep angles between 50 and 60 degrees. For the first group, the '032 patent indicates that some form of a mechanical leading-edge high-lift device (such as a slat) is required in order to avoid premature low-speed buffet. The addition of a mechanical leading-edge high-lift device avoids premature boundary-layer separation, alleviating the buffet problem, but it adds profile drag, weight, complexity, and cost. Under some circumstances, these disadvantages may outweigh the benefits of the raked wingtip. For the second group, the '032 patent indicates that the wingtip leading-edge sweep is great enough to trigger the formation of a stable leading-edge vortex, and that therefore premature buffet will not occur and no high-lift mechanisms are required.
The inventors herein have discovered that under some circumstances, leading-edge sweep angles of 50 to 60 degrees may not be adequate to ensure the formation of a stable leading-edge vortex when conventional transonic airfoils are used for the raked wingtip geometry. As used herein, "transonic airfoils" are those designed to operate at high subsonic freestream Mach numbers, with significant regions of locally supersonic flow. Additionally, even if the presence of a stable leading-edge vortex prevents premature buffet, such a vortex may result in higher drag than if the majority of the raked wingtip boundary layer could be kept attached over the range of typical operating conditions. Further, the technical viability of any raked wingtip would be improved greatly if there was no requirement for a leading-edge high-lift mechanism.
Thus, a need exists for an improved raked wingtip, particularly for use with aircraft that operate at high subsonic Mach numbers. The ideal raked wingtip would provide the aerodynamic benefits of an increase in wing span, while avoiding premature boundary-layer separation under high-lift conditions. Further, the optimal arrangement would not add significantly to wing weight or wing complexity. The present invention is directed to fulfilling this need.